CHAPTER XII. 

 ANIMAL HEAT. 



FROM the point of view of their temperature, animals fall into two 

 groups, namely (1) poikilothermic, or cold-blooded animals, whose 

 temperature varies with that of their surroundings, and (2) homoio- 

 thermic, or warm-blooded animals, whose temperature remains constant 

 except for slight daily variations, and is independent of that of their 

 surroundings. 



To the former group belong fishes and amphibia, to the latter birds 

 and mammals, including man. In man the normal temperature is 

 37 C. (984 F.); it shows a daily variation of approximately 1 F., 

 being highest in the afternoon and lowest in the early morning. It 

 is lowered by starvation or prolonged lack of sleep, and is raised by 

 muscular exercise. The constancy of the temperature is due to the 

 fact that heat production and heat loss balance each other. 



Production of Heat. The chemical Changes in the body which 

 constitute metabolism involve the production of heat, and this takes 

 place chiefly in the muscles, and to a smaller extent in the liver and 

 other glandular organs ; the blood leaving the liver, for instance, is 

 warmer than that entering it. The production of heat in the glandular 

 organs depends mainly on the variations of their activity associated 

 with the digestion of food, and is comparatively constant from day to 

 day. The heat formed in the muscles, however, varies greatly, being 

 enormously increased by muscular exercise ; and in warm-blooded 

 animals the amount of heat formed in the body depends largely upon 

 changes in the activity of the skeletal muscles. As has been pointed 

 out (p. 29), the greater part of the energy set free during muscular 

 contraction appears as heat. 



The heat formed in the body can be measured by means of a 

 calorimeter, the most suitable form of which for man is the Atwater- 

 Benedict calorimeter (p. 340). 



Loss of Heat. Heat is lost from the body principally through the 



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